Simpson posted a photo of herself looking svelte in a form-fitting Alexander McQueen dress from her new ad for Weight Watchers. "First time rocking a white dress this year ... but not the last!!Thanks @weightwatchers!" she captioned the picture. Simpson then shared a glam selfie and added: "... AND a white T-shirt!! #goalreached thx @weightwatchers."

In the close-up snapshot, Simpson strikes a sexy pose in a tight white T-shirt that shows off her toned arms. With her hair up in a bun, Simpson shows off her slim face.

The fashion mogul is mom to daughter Maxwell, 21 months, and son Ace, 7 months, with fiance Eric Johnson. Feeling good about her post-baby body, Simpson also shared adorable photos of her children on Instagram Thursday.

The proud mama captioned a close-up photo of her beautiful blonde daughter pouting her big full lips. "The most beautiful girl in the world," Simpson captioned the photo.

She also shared a photo of herself giving her son a big hug while smiling for the camera. "The most handsome boy in the world," she wrote. Little Ace looks adorable in the picture, wearing blue pajamas festooned with white whales.

"This body made two amazing little human beings. I love this body and what it's capable of -- no matter what size," Simpson said in an earlier Weight Watchers ad. "But this version feels really good."

Simpson is set to wear white again later this year, when she and Johnson finally plan to tie the knot. "We don't have an actual date but we do know that we want it to be this year," Simpson explained on "Good Morning America" on Jan. 7. "We need to get married."